我是初学的，大家帮忙看看吧，谢谢了。
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENtiTY ca2 IS
          PORT (clk : IN std_logic;
               Q1,Q2,Q3,Q4: OUT std_logic);
END ca2;
ARCHITECTURE behavioral OF ca2 IS
     COMPONENT dff
     PORT (
         d,clk :IN STD_LOGIC;
         q:OUT STD_LOGIC );
     END COMPONENT;
     SIGNAL data :STD_LOGIC_VECTOR (7 DOWNTO 0);
BEGIN   
     dffn_1: dff PORT MAP (data(0),clk,data(4));
     dffn_2: dff PORT MAP (data(1),clk,data(5));
     dffn_3: dff PORT MAP (data(2),clk,data(6));
     dffn_4: dff PORT MAP (data(3),clk,data(7));
     data(0) <= data(4) xor data(5);
     data(1) <= data(4) xor data(6);
     data(2) <= data(5) xor data(6) xor data(7);
     data(3) <= data(6);
     Q1 <= data(4);
     Q2 <= data(5);
     Q3 <= data(6);
     Q4 <= data(7);
END behavioral;